A rolling bearing or other parts which are liable to rusting are provided with a proper seal or other mechanical countermeasures to prevent the entrance of rusting substances. In automobiles, for example, bearings are disposed at positions which are not directly exposed to muddy water or mudguards are provided to prevent the entrance of rusting substances. Further, a dust-proofing seal plate is used to prevent the entrance of rusting substances into the interior of bearings.
However, a rolling bearing cannot be completely sealed because of its mechanism and thus normally is lubricated with a wetting grease provided with rust-proofing properties.
The provision of a grease with rust-proofing properties is normally accomplished by adding a material called rust preventive to the grease. As such a rust preventive there is widely used a chromate, nitrite, molybdate, tungstate or the like, particularly an inorganic passivator made of sodium salt thereof from the standpoint of its high performance. In particular, sodium nitrite is most effective and thus is most widely used. However, such an inorganic passivator is water-soluble and thus can hardly be dispersed in an oil-based material such as grease. Thus, a grease comprising a surface active agent in combination with such an inorganic passivator is commercially available as well.
Further, an organic rust preventive is used as well. Such an organic rust preventive is also called lipophilic organic inhibitor. Representative examples of such an organic rust preventive include sulfonate and carboxylate. However, this lipophilic organic inhibitor cannot itself exhibit rust-proofing properties so strong as the foregoing inorganic passivator and leaves something to be desired in rust-proofing properties particularly when exposed to saline. Thus, there is provided a grease comprising such a lipophilic organic inhibitor in combination with an inorganic passivator and optionally a surface active agent. This grease comprising an oil-soluble organic inhibitor in combination with an organic passivator and optionally a surface active agent exhibits better rust-proofing properties than a grease comprising an inorganic passivator alone.
However, sodium nitrite, which is a representative example of inorganic passivator, exhibits excellent rust-proofing properties but can affect animals and plants under some working conditions. It is thus said that the use of sodium nitrite should be avoided if possible.
On the other hand, stricter rust-proofing properties have been required of greases in recent years.
In the automobile industry, for example, as the use of automobiles diversifies as in running by the sea shore and running on the road sprinkled with salt for prevention of freezing, stricter rust-proofing properties have been required of greases. In the iron industry, too, improved rust-proofing properties have been required of greases, because the molding powder, which is used in continuous casting facilities, was changed to that which causes a part thereof to be dissolved in the cooling water, thereby accelerating rusting on bearings mounted in rolling mill.
In recent years, not only rust-proofing properties but also safety have been required of greases. However, there are no greases satisfying both the two requirements.